The present invention relates generally to automatic focusing systems, and is applicable particularly, but not exclusively, to video cameras.
Known is an automatic focusing system of a video camera which is based on the fact that the voltage level of a high-frequency component of the image signal corresponds to the degree of sharpness of the resultant image and in which the focusing control is performed so that the voltage derived as a focus voltage assumes the maximum level to bring the focus lens into the focus position. Such an automatic focusing system is disclosed, for example, in "NHK TECHNICAL RESEARCH REPORT", vol. 17 (No. 1), published in 1965, or "TELEVISION ACADEMY TECHNICAL REPORT ED", published in 1982 (November), wherein two focus voltages obtained in accordance with movement of the focusing lens in one direction along its optical axis are successively sampled and compared in level with each other and on the basis of a result of the comparison the focusing lens is controlled so as to take the focus position with respect to an object. Generally, in accordance with the movement of the focusing lens, the focus voltage is varied in level so as to draw a curve similar substantially to Gauss' curve with respect to the maximum focus voltage obtained when the focusing lens takes the focus position. Thus, the rate of change of the focus voltage level is not constant, that is, the rate of change is apt to be decreased in the lower level region and further in the higher level region. This provides a problem in that because the difference in level between the two focus voltages becomes too small in the lower and higher level regions, difficulty is encountered to accurately measure the difference between the two focus voltage levels and hence to insure high reliability in the automatic focusing control.